Striated muscle cells require a high degree of axial order to function properly. This FY the properties of the elastic titin filaments which link the ends of the thick, myosin-containing filaments to the nearest Z line were characterized. The central position of the thick filaments in the sarcomere, which is optimal for force generation, is intrinsically unstable. It was found that thick filament movement away from the center is slow on the physiological time scale. This slowness is due primarily to properties of the actomyosin crossbridges. The main role of the titin filaments in vivo is to keep the thick filaments centered during passive stretch of the muscle fiber and by recentering the thick filaments each time the muscle fiber is relaxed to prevent sarcomere asymmetry from adding up over several contractions. The influence of degradation of titin alone (by treatment with low concentrations of trypsin) or of titin plus nebulin (produced by high energy irradiation) on muscle structure was studied by X-ray diffraction. Both procedures caused the intensities of the equatorial reflections to decrease in relaxed fibers. Enzymatic treatment of rigor fibers did not affect the X-ray pattern, although irradiation caused the X-ray pattern to weaken. Electron microscopy suggests that both treatments bring about a reduction in thick filament diameter, which indicates that titin is attached to the outer surface of the thick filaments. Cryomicroscopy and image processing were used to obtain the axially projected mass distribution in relaxed and rigor rabbit muscle, and the equatorial X-ray diffraction pattern was calculated. The calculated intensities of the 10 and 11 reflections were close to the directly measured results, which makes it likely that the cryo and image processing techniques are free of artifact. A new imaging X-ray detector was characterized. It has been used to record good diffraction patterns from striated rabbit muscle in less than one tenth the time needed under similar conditions using film.